01 April 2015 (online)
(Triphenylphosphine)gold(I) chloride, ClAuPPh3, is a well characterized colorless complex with a melting point of 236–240 °C and it is soluble in most common organic solvents. Although it is commercially available, it is cheaper to prepare it directly from elemental gold in an easy, high-yielding two-step synthesis. In the first step, gold is dissolved in boiling aqua regia to form the gold(III) intermediate AuCl4 in solution. By adding dimethyl sulfide, the complex ClAuSMe2 precipitates as a white solid in an excellent yield of 93%.
This precursor complex is dissolved in dichloromethane together with triphenylphosphine to form immediately the desired complex in a very good yield of 92% after precipitation by adding methanol. ClAuPPh3 is a useful reagent in various types of reactions. It is often used in gold catalysis where it can act as co-catalyst or catalyst itself, or it can be used for the formation of more complex catalytic systems. ClAuPPh3 can also be used for the synthesis of organogold compounds, which can perform cross-coupling reactions in a generally mild manner and with high tolerance towards functional groups. Herein, some applications of ClAuPPh3 as reactant will be presented.
(A) ClAuPPh3 can be used for the generation of gold nanoparticles. In the first step, gold chloride reacts with an ethylene glycol silver carboxylate in a transmetallation reaction. The resulting gold(I) complex generates the gold nanoparticle by thermal induction. The resulting nanoparticles do not need any further stabilizing or reducing reagents and have size diameters of 3–6 nm with narrow size distribution.
(B) Zhang et al. used ClAuPPh3 as catalytic species for the formation of various saturated substituted O- and N-heterocycles. The homogeneous oxidative functionalization of terminal alkenes leads to a cyclization reaction that proceeds within short reaction times and under mild reaction conditions. In mechanistic studies they found that a conversion of C(sp3)-Au bonds into C(sp3)-C(sp2) bonds is catalyzed by an Au(I)/Au(III) system in a cross-coupling manner when using boronic acids as nucleophiles with Selectfluor as oxidant.
(C) Pérez-Sestelo and co-workers prepared different aryl-, alkynyl-, alkenyl-, and alkylgold(I) compounds in very high yields using the corresponding lithiated organic species and ClAuPPh3 as reagent. The organogold(I) compounds are used as nucleophiles in palladium-catalyzed cross-coupling reactions with various electrophiles under mild reaction conditions and in short reaction times.
(D) Meyer et al. showed that trimethyltin-substituted naphthalene derivatives perform transmetalation reactions under very mild conditions using ClAuPPh3 as reagent. One example showed the synthesis of a compound containing both a gold(I) moiety and an iodide function on the same molecule, which cannot be prepared using organolithium or Grignard reagents in this case.
(E) Rominger et al. used boronic acid derivatives as precursors for the synthesis of organogold(I) phosphane complexes. They are prepared in good yields of 82–98% using ClAuPPh3 as reagent. This method allows a higher tolerance towards functional groups than using lithiated species as reactants. As vinyl-, aryl- and heteroarylgold compounds are assumed to be intermediates in homogeneous gold catalysis, Rominger et al. used the prepared compounds to obtain more information about the mechanism of a catalytic cycle with gold.
(F) Blum and co-workers prepared vinyl and aryl organogold(I) compounds by treating the corresponding vinyl- or arylmagnesium bromides with ClAuPPh3. These simple compounds were used for continuing steps of reactions: First they performed a regio- and diastereoselective palladium-catalyzed syn-carboauration of alkynes. In a further step, di- and trisubstituted olefins were synthesized by either performing palladium-catalyzed cross-coupling reactions or electrophilic trapping reactions. These reactions demonstrate the potential of the combination of gold and palladium in organic synthesis.
(G) Keter et al. synthesized phosphinogold(I) dithiocarbamate complexes by using ClAuPPh3 and similar gold(I) precursors and different potassium salts of the corresponding dithiocarbamates under mild conditions and in short reaction times. The resulting complexes were tested for their activity against human cerival epithelioid carcinoma (HeLa) cells, a type of cancer. The P-Au-S moiety seemed to play an important role for the activity.
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